This invention relates generally to thermal management of electronic ballasts for fluorescent and other ballast-driven lamps, and more particularly to such thermal management using a piezoelectric fan, and to a piezoelectric fan especially suited to cooling electronic components and devices, including electronic ballasts.
Use of ballast-driven lamps is limited in some lighting applications by thermal considerations. For example, the problem of heat removal from compact fluorescent lamps with integrated ballasts, particularly those of higher wattage (e.g., 32 W and greater), has limited their use. Similarly, the use of ballast-driven fluorescent lighting in high hat and other closed luminaire applications has been limited by thermal considerations.
While the use of fans in cooling applications is, of course, relatively widespread, and different types of fans, including piezoelectric fans, are known for use in various cooling applications and other applications, to the inventors""knowledge, fans have not heretofore been integrated with electronic ballasts for lamps to cool the ballasts.
For example, U.S. Pat. No. 5,008,582 and Russian Patent No. 1,540,050 disclose the use of a piezoelectric fan for cooling a semiconductor chip or a microelectronic device, and U.S. Pat. Nos. 5,381,950 and 4,751,713 disclose the use of piezoelectric fans for other applications. International Publication No. WO 94/24437 discloses a piezoelectric pump. U.S. Pat. No. 4,283,658 discloses the use of a blower to cool a projection lamp. UK patent publication GB 2184302A discloses the use of a fan motor winding of a cooker or refrigerator as ballast for a fluorescent lamp within the cooker or refrigerator.
None of the patents identified above addresses the problem of heat management in ballast-driven lighting applications, and none discloses the use of a fan to cool an electronic ballast for a lamp. Although piezoelectric fans are known for cooling electronic components, as disclosed in the patents discussed above, there is a need for a compact and efficient piezoelectric fan for removing heat from small hot spots in, on or adjacent electronic components.
It is an object of the invention to thermally manage electronic ballasts for lamps.
It is another object of the invention to actively remove heat from electronic ballasts.
It is another object of the invention to provide a piezoelectric fan for localized or spot cooling at high efficiency.
It is another object of the invention to provide a compact piezoelectric fan having a low profile.
The invention disclosed herein addresses limitations on the use of ballast-driven lamps in thermally-sensitive applications, and achieves certain of the objects described above by actively removing heat from an electronic ballast, generally, or from one or more selected critical components therein. Such active heat removal from electronic ballasts is implemented in accordance with the invention by a fan.
In a preferred embodiment, the fan is a piezoelectric fan integrated with the ballast, for example, mounted to the ballast housing or to a printed circuit board within the housing, or to a component within the housing. Piezoelectric fans are well known in the art and essentially are solid state devices which do not require a driving motor. According to the invention, the piezoelectric cooling fan is coupled to the ballast to receive driving power therefrom. The ballast may be conventional and may include an AC to DC converter and an inverter, as well as other elements such as a power factor correction circuit and a DC to DC converter coupled between the AC to DC converter and the inverter for control purposes. The mechanical resonance of the fan and the frequency of the power coupled thereto from the ballast are related, e.g., the frequencies may be the same or they may be harmonically related.
In one embodiment, the piezoelectric fan is coupled to be driven from the AC line input to the ballast. In another embodiment, the AC to DC converter (e.g., a rectifier) in the ballast provides a DC voltage with an AC ripple voltage which (the AC ripple voltage) is coupled to drive the piezoelectric fan. In other embodiments, the inverter provides an AC voltage which is coupled to drive a lamp and to drive the piezoelectric fan. The driving power for the fan may be coupled from any other suitable point in the ballast. With suitable power coupling from the ballast, a separate control circuit for the fan is not needed, and the use of a piezoelectric fan eliminates the need for a fan motor.
A suitable conversion circuit converts the power coupled from the ballast circuit (e.g., from the AC line voltage, or the AC ripple voltage, or the AC lamp driving voltage) to one suitable to drive the piezoelectric fan.
The invention also addresses the need for a compact fan for removing heat from small areas and hot spots in, on or adjacent electronic components, and provides a piezoelectric fan which satisfies this need and achieves certain of the objects described above. A piezoelectric fan according to the invention, rather than operating by cantilever action to create an air flow, includes a membrane vibrated about a central plane by one or more suitably positioned piezoelectric elements to create an air flow through one or more apertures in the membrane.
The membrane is mounted directly to the component, or on or adjacent the localized area to be cooled, e.g., on or adjacent a hot spot. The piezoelectric element or elements, which flex when an appropriate electrical signal is applied thereto, are mounted to the membrane to vibrate it about the central plane. The membrane is fixed about its periphery to a frame, for example, and the piezoelectric element or elements are configured and fixed to the membrane at one end thereof to undergo relatively large out-of-plane bending movements at the free end thereof so as to vibrate the membrane normal to its plane. Such vibrations of the membrane cause air to be moved through a hole or holes in the membrane alternately in opposite directions. The moving air thus comes into direct contact with the hot spot adjacent the membrane to cool the hot spot.
Because the membrane vibrates about a central plane, the piezoelectric fan may be made compact and have a low profile, which makes it especially suited to removing heat from small hot spots in electronic circuitry or components.
In a preferred embodiment, four piezoelectric elements are positioned about the periphery of the membrane spaced 90 E apart, and the membrane is provided with a central aperture through which the air is moved. The membrane may be made of a polyamide material and the piezoelectric elements may be bimorphous ceramic elements.
The invention improves the state of the art by enabling air movement only near those components that need cooling. Thus, management of air movement is improved, which leads to more efficient heat removal. Moreover, the membrane, spot-type piezoelectric fan provided by the invention can be mounted directly to an electronic component as an integral part thereof. Thus, air impinges directly on the surface of the component, leading to very efficient heat removal. As mentioned, another advantage is the very low profile of the fan which is needed in miniaturized circuits.